Maintaining a stable borehole is one of the major problems encountered in drilling oil and gas wells. Hole instability is evidenced by the squeezing of soft, ductile formations into the borehole, the spalling under stress of hard, brittle formations, and the slumping and caving of shales, with consequent hole enlargement, bridges of cavings, and fill during trips into and out of the borehole. These problems increase the drilling time and thus the cost of drilling a well, and may result in stuck drill pipe and sidetracked borehole.
The various forms of hole instability resulting from the interaction between the drilling fluid and the subterranean formations penetrated by the borehole are believed to be related to the hydration and dispersion of the argillaceous sediments. Water is sorbed on clays by two mechanisms; adsorption of monomolecular layers of water on the planar surfaces of clay crystal lattices (commonly referred to as crystalline swelling or surface hydration), and osmotic swelling resulting from the high concentration of ions held by electrostatic forces in the vicinity of the clay surfaces. Crystalline swelling is exhibited by all clays. The water is strongly held but the increase in bulk volume is comparatively small. Interlayer osmotic swelling occurs only with certain clays of the smectite group, and causes large increases in bulk volume.
Where argillaceous sediments are compacted by the weight of overlying sediments, water adsorbed clay minerals is expelled along with pore water. The amount of water remaining in the subsurface sediments depends on the depth of burial; the species and amounts of clay minerals present in the sediment; the exchange cations thereon; and the geologic age of the formation. When the stable is penetrated by the bit, the horizontal earth stresses on the walls of the hole are relieved and the shale adsorbs water from the drilling fluid. If the swelling pressure thus developed increases the hoop stress above the yield stress, the hole is destabilized. This destabilization takes the form of plastic yielding when predominately sodium montmorillonite sediments are contacted by fresh water fluids.
Caving and hole enlargement are frequently experienced in the older, consolidated shales that contain no montmorillonite. It has been shown that these shales can develop extremely high swelling pressures when confined and contacted with water. This swelling pressure increases the hoop stress around the borehole. When the hoop stress at the wall of the hole exceeds the yield stress of the shale, hydrational spalling occurs. It has often been observed that severe caving does not occur until several days after the shale is penetrated by the bit.
Since borehole hydration is, in many cases, the prime cause of hole instability, and in many other cases a contributing factor, every effort must be made to control it.
Laboratory tests have shown that soluble salts must be present in the aqueous phase of water base drilling fluids to reduce the repulsive forces between the clay surfaces, thus reducing the swelling of the clay. Both field and laboratory investigations show that potassium salts are more effective at repressing swelling and dispersion than an equivalent amount of sodium salts. Potassium chloride is the most commonly used potassium salt. The concentration of potassium chloride required to prevent swelling depends on the nature of the shale and on the clay minerals present.
A disadvantage of water base drilling fluids is their tendency to disperse clay formations and drill cuttings, with consequent hole enlargement and accumulation of drilled solids in the mud. Although dispersion is closely associated with swelling, the two phenomena are not identical. Hard consolidated shales can develop high swelling pressures but have little tendency to disperse. Swelling and dispersion of shales have long been inhibited by the use of muds containing lime or gypsum, and thinners--such as tannates or chromelignosulfonates--to offset the flocculating effect of the calcium ion. Although these muds cause much less swelling and dispersion than do sodium muds, they require frequent maintenance treatments, and the sodium hydroxide required to solubilize the thinner partially offsets the inhibiting effect of the calcium ion. Furthermore, because of their relatively high solids content, they retard penetration rates.
Because of these disadvantages, a class of muds known under the general title of low solid or non-dispersed muds have come into increasing favor. These muds rely on polymers and soluble salts to inhibit swelling and dispersion, and on the extensive use of mechanical separators to prevent the accumulation of drilled solids. Usually no thinners are added, and the pH is kept as low as is consistent with the control of corrosion. Commonly used polymers are cellulose derivatives, starch derivatives, polyacrylamide-acrylate copolymers, and xanthan gum. The liquid phase may be potassium, sodium or calcium chloride brine, sea water, or fresh water with a few pounds per barrel of diammonium phosphate.
The mechanism by which the polymers stabilize shales is not known for certain. The commonly used polymers are anionic polyelectrolytes, and starches, which are presumed to be adsorbed on clay aggregates. The polymer bridges the particles together and prevents dispersion of the shale.
Since hole instability is a complex problem, the nature of which depends on the borehole environment, the type of drilling fluid that will provide maximum hole stability therefore varies from area to area.
It has been attempted to base the choice of drilling fluid on a classification of shales according to clay mineral composition and texture. Shales can be classified in a general way to describe their relative tendencies to swell and disperse. Smectite clays have a high tendency to swell and disperse. Illite clays have a much lower tendency to swell and disperse, while the interlayered clays are intermediate to smectites and illites. The swelling and dispersion characteristics of shales has been presumed to be a function of the amounts and types of clays present in the shales. However, the hydration and dispersion characteristics of shales may be considerably different than predicted by a mineralogical classification. Generally these characteristics are worse than predicted by the mineralogical compositions of the shale.
Thus there is a need to develop a method of determining the hydration and dispersion characteristics of the subterranean formations being drilled which will enable the proper inhibited aqueous drilling fluid to be used.